PROJECT SUMMARY Opioid abuse is an expanding national health epidemic. A central feature of addiction to opioids such as heroin, is the high incidence of relapse to drug use despite available treatments. Underlying this propensity to relapse are persistent neuroadaptations that emerge and develop during the course of drug use. The most profound of these adaptations occur within brain regions associated with mediation of reward and motivation. The nucleus accumbens (NAc) region is involved in the facilitation of drug reward, subsequent drug craving, and relapse to drug use. This region undergoes extensive drug-induced dysregulation, in part mediated by epigenetic mechanisms. Histone deacetylases (HDACs) are enzymatic regulators that can produce experience-dependent alterations in transcription and translation. Class II HDACs, including HDAC5, are dysregulated by drugs of abuse in the NAc and overexpression of HDAC5 can limit drug reward and drug-seeking behavior. When HDAC5 is localized to the nucleus, it is able to repress the transcription of many genes known to play an active role in the suppression of drug-seeking behavior. However, the crucial time-point at which HDAC5 is acting to suppress these behaviors, as well as the cell populations in which HDAC5 regulation is necessary are unknown. Furthermore, candidate differentially regulated genes that HDAC5 may be acting on to suppress heroin-seeking behaviors have not been investigated in in vivo drug self-administration models. The aims laid out in this proposal will identify when HDAC5 is acting, where within the NAc HDAC5?s effects are necessary, and how HDAC5 may be effecting the epigenetic landscape to limit heroin-seeking. These experiments employ a fusion of breakthrough transgenic lines with leading-edge viral vector constructs and modern next generation sequencing techniques. All analyses will be performed in a mouse model of the benchmark standard for substance abuse research: drug self-administration. The Medical University of South Carolina is a leading center in the substance abuse field and is an ideal location for the actuation of these experiments. The comprehensive experimental design of these studies and rigorous application of pioneering techniques will significantly address the gap in understanding of HDAC5?s ability to limit drug reward and provide significant learning and development opportunities that will serve as a career-building foundation.